go-smtp

An ESMTP client and server library written in Go.

Features

ESMTP client & server implementing RFC 5321
Support for SMTP AUTH and PIPELINING
UTF-8 support for subject and message
LMTP support

Usage

Client

package main

import (
	"log"
	"strings"

	"github.com/emersion/go-sasl"
	"github.com/emersion/go-smtp"
)

func main() {
	// Setup authentication information.
	auth := sasl.NewPlainClient("", "user@example.com", "password")

	// Connect to the server, authenticate, set the sender and recipient,
	// and send the email all in one step.
	to := []string{"recipient@example.net"}
	msg := strings.NewReader("To: recipient@example.net\r\n" +
		"Subject: discount Gophers!\r\n" +
		"\r\n" +
		"This is the email body.\r\n")
	err := smtp.SendMail("mail.example.com:25", auth, "sender@example.org", to, msg)
	if err != nil {
		log.Fatal(err)
	}
}

If you need more control, you can use Client instead. For example, if you want to send an email via a server without TLS or auth support, you can do something like this:

package main

import (
	"log"
	"strings"

	"github.com/emersion/go-smtp"
)

func main() {
	// Setup an unencrypted connection to a local mail server.
	c, err := smtp.Dial("localhost:25")
	if err != nil {
		return err
	}
	defer c.Close()

	// Set the sender and recipient, and send the email all in one step.
	to := []string{"recipient@example.net"}
	msg := strings.NewReader("To: recipient@example.net\r\n" +
		"Subject: discount Gophers!\r\n" +
		"\r\n" +
		"This is the email body.\r\n")
	err := c.SendMail("sender@example.org", to, msg)
	if err != nil {
		log.Fatal(err)
	}
}

Server

package main

import (
	"errors"
	"io"
	"io/ioutil"
	"log"
	"time"

	"github.com/emersion/go-smtp"
)

// The Backend implements SMTP server methods.
type Backend struct{}

func (bkd *Backend) NewSession(_ *smtp.Conn) (smtp.Session, error) {
	return &Session{}, nil
}

// A Session is returned after EHLO.
type Session struct{}

func (s *Session) AuthPlain(username, password string) error {
	if username != "username" || password != "password" {
		return errors.New("Invalid username or password")
	}
	return nil
}

func (s *Session) Mail(from string, opts *smtp.MailOptions) error {
	log.Println("Mail from:", from)
	return nil
}

func (s *Session) Rcpt(to string) error {
	log.Println("Rcpt to:", to)
	return nil
}

func (s *Session) Data(r io.Reader) error {
	if b, err := ioutil.ReadAll(r); err != nil {
		return err
	} else {
		log.Println("Data:", string(b))
	}
	return nil
}

func (s *Session) Reset() {}

func (s *Session) Logout() error {
	return nil
}

func main() {
	be := &Backend{}

	s := smtp.NewServer(be)

	s.Addr = ":1025"
	s.Domain = "localhost"
	s.ReadTimeout = 10 * time.Second
	s.WriteTimeout = 10 * time.Second
	s.MaxMessageBytes = 1024 * 1024
	s.MaxRecipients = 50
	s.AllowInsecureAuth = true

	log.Println("Starting server at", s.Addr)
	if err := s.ListenAndServe(); err != nil {
		log.Fatal(err)
	}
}

You can use the server manually with telnet:

$ telnet localhost 1025
EHLO localhost
AUTH PLAIN
AHVzZXJuYW1lAHBhc3N3b3Jk
MAIL FROM:<root@nsa.gov>
RCPT TO:<root@gchq.gov.uk>
DATA
Hey <3
.

Relationship with net/smtp

The Go standard library provides a SMTP client implementation in net/smtp. However net/smtp is frozen: it's not getting any new features. go-smtp provides a server implementation and a number of client improvements.

Licence

MIT